The present invention relates to an improved angle measuring device such as a transit (theodolite).
A device for measuring angles without using scales is disclosed, for instance, in U.S. Army Engineer Topographic Laboratories Report ETL-TR-72-1. In this conventional angle measuring device, a period of time T.sub.1 for a slit provided in the periphery of a rotor rotating at a constant speed to make one complete revolution starting from a stationary reference point and a period of time T.sub.2 required for the slit to move from the reference point to an angle measuring point are obtained, and the angle to be measured is obtained by evaluating the following expression: EQU 360.degree..times.(T.sub.2 /T.sub.1).
This angle measuring device is advantageous in that, as no scale is needed, the way in which the measurement is made is simple, and the construction of the device is also simple.
The above report also refers to a so-called "180.degree. opposed reading method" used to compensate for the error which is caused when the center of rotation of the rotor is not coaxial with the center of rotation of the angle measuring point. In the method disclosed by the report, the speed of the rotor greatly affects the angle measurement accuracy. Accordingly, in order to maintain a high measurement accuracy, the motor driving the rotor must be capable of providing a highly constant speed.
If one cycle of rotational irregularity occurs within each revolution of the rotor, its adverse effects may be cancelled by employing the 180.degree. opposed reading method. On the other hand, it has been found through experiments on the system disclosed by the report that, in a motor or in a feedback detecting system for rotation control, usually two cycles of rotational irregularity occur for each revolution. This error cannot be cancelled with the 180.degree. opposed reading method.